The Cosmos Is Cracked

If the prospect of an ever-expanding universe that eventually stretches into a vast emptiness isn’t depressing enough, there’s this: the universe may have cracks in it.

Cracks, called cosmic strings, are topological defects in spacetime that might have formed when the universe was young. Experiments haven’t found any proof that cosmic strings are even out there, but that hasn’t stopped physicists from calculating how many strings we might expect there to be if, in fact, they turn out to exist after all. And it’s a lot: cosmic strings would produce at least a billion loops throughout the visible universe today, researchers report. “Cosmic strings are these filaments which wind and sneak throughout the universe,” says study co-author Benjamin Shlaer of Tufts University. They are essentially one-dimensional fault lines in space, made not of mass but pure energy. Some could be infinitely long, and all are almost impossibly thin, much narrower than a proton.

Cosmic strings may have formed as the universe cooled after the big bang. Just as water undergoes a “phase transition” from liquid to solid when it freezes, the universe changed radically when it went through a transition from being extremely hot and dense in the instants just after the big bang to slightly cooler and more rarefied just a few fractions of a second later. According to a popular theory, in the hot and dense universe three of the four forces of nature (weak, strong and electromagnetic) were unified but in the cooler universe they separated. When this symmetry among the forces broke, it might have created topological defects in the form of strings, so named because they would be long, thin fissures in space. (Despite the similar names, cosmic strings may or may not be related to the strings predicted to make up fundamental particles in string theory .)

These strings would have started off tangled and wrinkly when the universe was in its hot, dense state but would have stretched out over time as space itself expanded. This movement would cause some strings to cross others. “When they wind back on themselves they break so that the wrinkles snap off as closed loops, like little rubber bands.” The loops are what astronomers might be able to detect because they would oscillate, producing measurable ripples in spacetime called gravitational waves.

Shlaer and his colleagues created a numerical simulation of cosmic string loop formation and ran it on a supercomputer cluster at the university. The results told them how big loops are likely to be when they form, and by extrapolating, the researchers calculated the number and size of the loops that might exist in the universe at any given time. The results depend on how taut the strings are—a property determined by the temperature of the universe when they were formed. For a likely range of tensions, the scientists calculated that billions of cosmic string loops could exist today. “The Tufts group has done a heroic job with the string simulations, and they pin down important features of the loop distribution critical for predicting gravitational-wave emission and their effects on millisecond pulsar timing,” says Tanmay Vachaspati, a physicist at Arizona State University in Tempe who wasn’t involved in the research.

The new study gives observers a better idea of what to look for in the quest to find evidence of cosmic strings. The strings would create gravitational waves that could be detectable by elaborate wave-detecting facilities such as LIGO (the Laser Interferometer Gravitational-Wave Observatory) in the U.S. or by studies of rapidly rotating stars called pulsars, which emit beacons of light with clockwork precision. If astronomers on Earth notice a change in the arrival time of light from pulsars, it could mean a gravitational wave has hit our planet. The fact that no evidence for gravitational waves has yet been found already eliminates the possibility of cosmic strings with a given range of tensions. Whether or not any cosmic strings exist is still an open question.